A common mode of failure for saw chains used on mechanical harvesters is rivet shear. The reason for such increased rivet shear is that tree harvester saw chain has been simply a larger version of saw chain suited for conventional chain saws. Tree harvesters, however, apply a significantly greater force in the saw chain, which in turn can cause a saw chain to bind in the bar groove, not release when engaging an un-cuttable object, and the like. Since conventional chain saw chain is not suited to withstand such forces, the tree harvester saw chains are prone to breaking, and in particular to shearing at the shoulder of the rivets coupling the chain components together.
Once broken, the end of the chain can be rapidly accelerated in a whip-like motion wherein other parts of the chain may break free, and fly through the air with as much kinetic energy as a rifle bullet. This phenomenon is referred to as chain shot. Of course, chain shot is dangerous to persons, and equipment, nearby. Steps to reduce risk to operators and equipment include, chain catchers, chain shot guards, and replacing the standard 13-mm cab glass with 19-mm or thicker laminated polycarbonate windows. Other steps to mitigate risk include inspecting chains for damage before use. However, it is believed that many chains fail the instant they are damaged.
Saw chains for concrete cutters, for example, may also tend to break through the rivets and rivet holes as the chain material contacting the bar is worn away. To provide longer life to the chain more material can be added between the bar contact area and rivet hole by reducing the rivet hole diameter in the cutters and tie straps. This added material can increase the strength and life of the cutters or tie straps but decreases the shearing strength of the rivets because the rivet diameter is reduced. Striking a balance between rivet diameter and material thickness in the other chain components may be difficult.